Due to their hydrocarbon nature, many of the synthetic polymers widely used today are also undesirably flammable. In view of their widespread use, this characteristic has contributed to a significant fire hazard, and as a consequence, there have been many attempts in the past to increase the flame resistance of such polymers. In order to minimize the fire potential, for example, chemicals recognized for their ability to impart fire retardancy have often been mechanically incorporated with hydrocarbon polymers to form polymer compounds resistant to fire. Fire retardant compounds containing phosphorus, for example, have frequently been used for this purpose. While such compounds have proven effective in increasing the flame resistance of hydrocarbon polymers in which they are present, homogeneous mixtures of the retardants in polymers are often difficult to prepare. In addition, the incorporation of fire retardants in the polymers requires an additional processing step, that of mixing, which is both time consuming and expensive. In order to avoid such drawbacks in polymerization reactions that proceed by free radical mechanisms, there have been attempts to find organo-phosphorus compounds capable of participating directly in the polymerization reaction itself. For example, organo-phosphorus compounds have been successfully employed as monomers; as chain transfer agents, and as polymerization initiators. Such attempts have disclosed that some phosphoric acid and phosphinic acid derivatives having vinyl or alkyl substituents therein, for instance, are capable of polymerizing or copolymerizing in the presence of free radical initiators. Toy, Chem. Eng. News, 25, No. 28, 2030, 1947.
In addition, organo-phosphorus compounds have been used as comonomers to incorporate phosphorus atoms into the backbone of polymers, and in this connection, a number of monomers have been polymerized in the presence of phosphorus compounds having the formula RPX.sub.2, where R is organic, and X is chlorine or bromine; U.S. Pat. Nos. 2,671,077; '078; '079; and '080.
Apart from their ability to impart flame retardancy in polymers in which they are incorporated, a further desirable characteristic of phosphorus compounds containing PH and PX bonds is their ability to act as chain transfer agents. In such capacity, they are useful in achieving molecular weight control, as well as for the purpose of reducing unwanted cross-linking. The utility of trivalent phosphorus compounds having labile hydrogens on free radical polymerizations, for example, has previously been studied, Pellon, Journal of Polymer Science, 43, 537, 1960.
A still further use for phosphorus-containing compounds in free radical polymerizations is in the capacity of photoinitiators. In this regard, the acylphosphine oxides and related compounds have been employed as effective photoinitiators for various unsaturated systems; Baxter & Davidson, Polymer, 29, 1569, 1988. For instance, the acylphosphine oxides have a relatively strong absorption in the 300-400 nm region which enables them to absorb U.V. light at the point where pigments such as titanium dioxide are relatively transparent.
In view of the foregoing, therefore, it is a first aspect of this invention to provide improved phosphorus compounds that are useful as flame retardants for hydrocarbon polymers.
A second aspect of this invention is to provide pentavalent phosphorus compounds that function to incorporate higher levels of phosphorus as an integral part of polymer chains.
Another aspect of this invention is to provide bi(phosphine) compounds that serve as chain transfer agents.
A further aspect of this invention is to provide bi(phosphine) compounds that yield radicals upon photolysis which possess a high degree of polymerization-initiating capability.
Another aspect of this invention is to provide bi(phosphine) compounds that enhance the thermal stability of hydrocarbon polymers in which they are physically incorporated.
An additional aspect of this invention is to provide phosphorus-containing vinyl polymers having a greater affinity for metal surfaces.
Yet another aspect of this invention is to provide phosphorus-containing vinyl polymers possessing improved thermal and oxidative stability.
A still further aspect of this invention is to provide a pentavalent phosphorus compound that yields two phosphorus-containing radicals upon its decomposition.